The abrupt extinguishing of any life before it reaches a natural apogee is an irresistible occasion for speculation about what might have been. When the life was that of a formidable politician with a large and passionate following, such speculation becomes an important civic act, clarifying the nation's trajectory and the possibilities that may have been closed by the person's death. For American liberals, the past 35 years seem haunted by sad hypotheses that linger in the air, like echoes of three silenced voices.

Thirty years ago the assassination of Robert F. Kennedy, then 42, occurred less than five years after the assassination of his 46-year-old brother, the President, and less than two months after the assassination of the 39-year-old Martin Luther King Jr. By a total of just six bullets fired in Dallas, Memphis and Los Angeles, all Americans should have been somewhat immunized against historicisms that disparage the importance of contingencies in deflecting the direction of the human story.

Now comes Michael Knox Beran to further trouble the equanimity of liberals, and tantalize the imaginations of everyone else. In ''The Last Patrician: Bobby Kennedy and the End of American Aristocracy,'' Beran, a New York lawyer, argues that what Sirhan Sirhan ended the night Kennedy won the California primary was not a reinvigoration and restoration of Rooseveltian liberalism. Rather, he believes, the loss was larger than that because Kennedy was by then a more original force than liberals understood or can yet comfortably acknowledge. If Beran is correct, what was stopped in the kitchen of the Ambassador Hotel was Kennedy's unenthralled reassessment, tantamount to rejection, of what liberalism then was. Kennedy's criticism, as Beran tickles it from his words and deeds, looks like a close cousin of conservatism.

But is Beran correct? That is difficult to determine, because his argument is occasionally chaotic -- which makes his book more stimulating than convincing. However, he is such a lively writer, and such a risk-taking thinker, that the sparks he promiscuously strikes from his literary flint are, cumulatively, illuminating. And what he says is clear enough that it will provoke liberals to accuse him of an intellectual train robbery. They have suffered enough, not least at the hands of the current President, and surely are in no mood to endure quietly a claim by a rival sect to their last saint.

''There has been no rush to tell the truth about Bobby Kennedy,'' Beran declares, because ''in his guise as the last great liberal, Bobby is a valuable commodity, a rare example of a liberal icon whose appeal remains undiminished today.'' However, the truth as Beran tells it is not as dramatic as he suggests.

He says, in his occasionally overheated way, that Augustine was a pagan before he was a saint, that Paul was a Jew before becoming a revolutionary Christian, that Luther was a priest before becoming a Protestant, and Kennedy . . . well, really. Beran acknowledges that as late as the beginning of 1965, Kennedy was ''a most conventional statesman.'' What happened in his last three years to justify comparisons with the radical transformations of Augustine, Paul and Luther? Not all that much, as measured either in words or in deeds.

Granted, particularly in his last campaign, the 1968 California primary contest with Hubert Humphrey and Eugene McCarthy, something -- evolving convictions or short-term calculations, or both -- caused him to seem the least liberal of the three. And he had already been criticizing the welfare state as a blunt weapon against poverty and a dehumanizing instrument of patrician condescension toward the lower orders. But he simultaneously found fault with President Lyndon B. Johnson for not requesting funds for this and that portion of the welfare state more generously.

Still, about a month before the California primary, a headline in The New York Times announced, ''Kennedy: Meet the Conservative.'' And California's Governor, Ronald Reagan, genially said, ''I get the feeling I've been writing some of his speeches.'' In the turmoil of the overlapping foreign and domestic crises of the mid-60's, Kennedy did seem to become more than a runner to whom other people's batons had been passed. But even then his brief excursions into new ideological country consisted only of suggestive remarks and tentative actions, not sustained analyses and settled programs.

Beran is at his considerable best in tracing Kennedy's path to what Beran thinks was his disillusionment with a liberalism that had become ''a curiously regressive phenomenon: though it ostensibly celebrated the Forgotten Man, it in fact trivialized and diminished him.'' That course was charted by Joseph Kennedy's cold-blooded and long-headed assessment of the revived patrician class, into which he was determined to insert his sons.

Beran believes that in late-19th-century America, aristocracy ''meant weakness; it meant effeminacy; it meant Henry James.'' But the first half of the 20th century brought the ''risorgimento of the well-to-do.'' With the sort of muscular assertion that makes Beran's book grip any argumentative reader, he says, ''The revolt of the blue bloods against their consignment to an historical oblivion of poetry and country houses began on the day when the young Teddy Roosevelt, bullied 'almost beyond endurance' by two boys whom he had neither the strength nor the skill to subdue, decided to take up boxing and learn how to fight back.'' It was a straight line to the ''aristocracy of action'' and football on the Hyannis lawn.

This was, Beran says, ''one of the more astonishing comebacks in the history of American political power. Thrown out by Jefferson at the beginning of the 19th century, the patricians returned to power at the beginning of the 20th, and the second time they did not make the mistake of alienating the common man.'' But they did not respect him. ''Franklin Roosevelt,'' Beran insists, ''perfected the technique of enveloping the lower classes in a warm rhetorical bath of patrician solicitude.'' And: ''The two Roosevelts made a great show of trying to help the Common Man, the Forgotten Man, the Average Man, but they never made any pretension to being, like Jackson and Lincoln, like Eisenhower and Reagan, common men themselves.''

BERAN contends that Bobby Kennedy was different, that he had (to use a term that was in vogue when he was) authenticity. Having had two brothers, a sister and a brother-in-law die violently, he acquired a genuine identification with suffering, particularly that of what has come to be called the underclass. If so, liberal thinkers were slow to recognize his aptitude for empathy. When in 1964 he moved to New York (he lived in the same building as Truman Capote; try to imagine their small talk in the elevator) to run for the Senate against the moderate Republican incumbent, Kenneth Keating, such liberal authenticators as Archibald MacLeish, Richard Hofstadter, Barbara Tuchman and the N.A.A.C.P. supported Keating. Kennedy won but ran more than a million votes behind President Johnson in the election.

And six months later he was saying, regarding poverty policy, ''We have gone as far as good will and even good legislation will take us.'' He soon plunged, against the advice of the prematurely sobered, including Daniel Patrick Moynihan, into saving Brooklyn's Bedford-Stuyvesant section. It was to be done by -- how anachronistic the phrase now seems -- ''urban renewal.'' But it was to be renewal from the bottom up, renewal by ''participation'' that would produce a true ''community,'' which in turn would heal the souls of ghetto residents.

Kennedy did solicit capital and expertise from the traditional patrician sources. The key to his respectful liberalism was to be reliance on the untapped skills and spirit of the community. The problem was that if tapping this supposed local reservoir of virtues -- this social capital of talents -- were all that renewal required, the ghetto would have been basically healthy and would not have needed much renewing.

STILL, this was a time when Lewis Mumford was testifying to Congress that ''democracy, in any active sense, begins and ends in communities small enough for their members to meet face to face.'' The historians J. G. A. Pocock, Bernard Bailyn and Gordon Wood were emphasizing the extent to which classical understandings of citizenship influenced the American Founders. Furthermore, Beran argues that by passing through the furnace of family tragedy, Kennedy had become both Greek and Emersonian. After his brother was murdered, Jacqueline Kennedy gave him a copy of ''The Greek Way,'' by Edith Hamilton, and that deepened his thinking about the meaning of civic life. Also, reading Emerson fueled his interest in what Beran calls ''the problem of the underconfident soul'' -- the conditions that cripple the growth of confidence, a prerequisite for success in an urban setting, and in a world increasingly ruled either by impersonal market forces or by faceless bureaucracies.

If Beran reads Kennedy's career correctly, what was at least latent in his thinking was indeed a harbinger of today's problematic emphasis on therapeutic government's supposed duty to deliver ''values'' as well as the mail. That this ambitious agenda for government is central to today's conservatism (liberalism's version stresses ''self-esteem'' as a progressive aim of public policy) is among the paradoxes of contemporary politics.

Twenty-eight years were to pass between Kennedy's death and substantial reform of the welfare state. When reform came, it involved repeal of a 61-year-old entitlement (Aid to Families With Dependent Children) and the largest devolution of power from the Federal Government to the states since the end of Reconstruction. The aims of some state experiments now under way include the revitalization of intermediary institutions between individuals and government, and a quickening sense of urban community. Beran dismisses ''community'' as political ''snake oil,'' stressing that families are the incubators of successful citizens. But Kennedy was right: families cannot be severed from connection with communities, with their schools and churches and jobs and with their drug dealers, liquor stores and gangs.

Beran's slender meditation on Kennedy's truncated life has an unusually high ratio of provocations per page. Some readers will angrily throw it across the room. But they will retrieve it, and continue reading, avidly.